Power harvesting in a passive rfid device

ABSTRACT

A method of harvesting power in a passive RFID device including a passive biometric authentication engine, including the steps of receiving a command from a powered RFID reader and supplying power extracted from an excitation field of the RFID reader to the biometric authentication engine whilst the RED reader waits for a response to the command. Responsive to determining that a period that the RFID device has been waiting for a response exceeds a predetermined threshold, and that a process being performed by the biometric authentication engine has not been completed, a request for a wait time extension is sent to the RFID reader to cause it to continue supplying the excitation field.

The present invention relates to power harvesting in an RFID device, andparticularly to power harvesting in a passive RFID device includingadditional components requiring power such as a fingerprint scanner.

FIG. 1 shows the architecture of a typical passive RFID device 2. Apowered RFID reader 4 transmits a signal via an antenna 6. The signal istypically 13.56 MHz for MIFARE® and DESFire® systems, manufactured byNXP Semiconductors, but may be 125 kHz for lower frequency PROX®products, manufactured by HID Global Corp. This signal is received by anantenna 8 of the RFID device 2, comprising a tuned coil and capacitor,and then passed to an RFID chip 10. The received signal is rectified bya bridge rectifier 12, and the DC output of the rectifier 12 is providedto a control circuit 14 that controls the messaging from the chip 10.

Data output from the control circuit 14 is connected to a field effecttransistor 16 that is connected across the antenna 8. By switching onand off the transistor 16, a signal can be transmitted by the RFIDdevice 2 and decoded by suitable control circuits 18 in the reader 4.This type of signalling is known as backscatter modulation and ischaracterised by the fact that the reader 4 is used to power the returnmessage to itself.

As an additional security measure, some RFID devices have been adaptedto additionally process biometric identification data to provideimproved security. In such systems, the user is provided with an RFIDcard having a biometric template stored on it. A terminal, for exampleto enable the owner of the card to gain access to money or physicalaccess to a building or office, is provided with a fingerprint sensorand, to authorise the user, a fingerprint read from the terminal istransmitted from the terminal to the RFID card, where a match isperformed with the stored template on the card. The RFID card thenwirelessly communicates to the terminal the results of the livematching, yes or no.

It is herein proposed to incorporate a biometric sensor, such afingerprint scanner, into a passive RFID device. At least the preferredembodiments of the present invention seek to solve some of the technicalproblems associated with such a device.

The present invention provides a method for harvesting power in apassive RFID device comprising a passive biometric authenticationengine, the method comprising: receiving, by the RFID device, a commandfrom a powered RFID reader; receiving, by the RFID device, a non-pulsingcontinuous radio-frequency excitation field whilst the RFID reader waitsfor a response to the command; harvesting, by the RFID device, powerfrom the excitation field; supplying the power extracted from theexcitation field to the biometric authentication engine; performing aprocess in the biometric authentication engine, the process being onenot required for responding to the command from the RFID reader;determining a period that the RFID device has been waiting for aresponse; and responsive to determining that the period exceeds apredetermined threshold if the process has not been completed, sending,by the RFID device, a request for a wait time extension to the RFIDreader.

As will be discussed in greater detail below, typical RFID readers pulsetheir excitation signal on and off so as to conserve energy, rather thansteadily emitting the excitation signal. Often this pulsing results in aduty cycle of useful energy of less than 10% of the power emitted bysteady emission. This may be insufficient to power a biometricauthentication engine.

The above method overcomes this problem by taking advantage of certainaspect of the standard functionality of a RFID reader complying with,for example, international standard ISO/IEC 14443. Particularly, whilstthe RFID reader waits for a response to a command, it must maintain anon-pulsing, preferably a substantially continuous, radio frequency (RF)excitation field.

Thus, in accordance with this method, when the RFID reader sends acommand to the RFID device, the device does not respond, but ratherwaits and harvests the power to drive the functionality of the biometricauthentication engine.

The process performed by the biometric authentication engine is one notrequired for responding to the command, for example the command may be a“request to provide identification code” command. That is to say, aresponse to the command from the RFID device is intentionally delayed soas to allow the processing to be performed.

In the preferred embodiments, the RFID device does not respond to thecommand whilst the biometric authentication engine is performing aprocess. Furthermore, the method preferably further comprises: after thebiometric authentication engine completes the process, responding by theRFID device to the command.

The steps of “determining a period that the RFID device has been waitingfor a response; and responsive to determining that the period exceeds apredetermined threshold if the process has not been completed, sendingby the RFID device a request for a wait time extension to the RFIDreader” are preferably repeated until the process is completed and/or aresponse to the command has been sent. For example, after the processhas been completed, the RFID device may allow the wait time to expire,if no further communication with the RFID reader is required.Alternatively, a response to the RFID reader may be sent, for example ifthe process was part of an authorisation step before responding to thecommand.

Preferably, the period is a time since the command was received or sincethe last wait time extension request was made. Thus, the request for await time extension can be sent before expiry of the current wait timeto ensure that the RFID reader continues to maintain the RF excitationfield until the process is complete.

The process performed by the biometric authentication engine may be oneof a biometric enrolment process or a biometric matching process. Thedescribed method is particularly applicable to biometric matching orenrolment, for example fingerprint matching or enrolment processes, asthese processes require input from the user (i.e. one or more biometricscans), which can only be processed at the rate that they are suppliedby the user of the RFID device.

Without using a request for a wait time extension, the maximum defaulttime that a non-pulsing RF excitation field could be supplied is 4.949seconds for an RFID reader complying with international standard ISO/IEC14443. Thus, the method allows processes to be performed by thebiometric authentication engine, wherein the process requires greaterthan 5.0 seconds to be completed.

In various embodiments, the biometric authentication engine may includea biometric scanner and a processing unit. Preferably, the biometricauthentication engine is a fingerprint authentication engine.

As discussed above, the present method is particularly applicable todevices and readers complying with international standard ISO/IEC 14443(although the method may be applicable also to other standards operatingin a similar manner), and thus the RFID device is preferably a proximityintegrated circuit card (PICC) and the RFID reader is preferably aproximity coupling device (PCD). The PICC and PCD preferably comply withthe definitions set forth in the international standard ISO/IEC 14443.

The predetermined threshold is preferably below a pre-arranged firstwait time of the PICC and the PCD.

Viewed from a second aspect, the present invention provides a passiveRFID device comprising: an antenna for receiving a radio-frequencyexcitation field from an RFID reader and for harvesting power from theexcitation field; a passive biometric authentication engine arranged toreceive power harvested by the antenna; and an RFID device controllerarranged to perform a method, comprising: receiving, by the antenna, acommand from a powered RFID reader; receiving, by the antenna, asubstantially continuous radio-frequency excitation field whilst theRFID reader waits for a response to the command; performing a process inthe biometric authentication engine, the process being one not requiredfor responding to the command from the RFID reader; determining a periodthat the RFID device has been waiting for a response; and responsive todetermining that the period exceeds a predetermined threshold if theprocess has not been completed, sending by the antenna a request for await time extension to the RFID reader.

The RFID device controller is preferably further arranged to perform anyor all of the preferred steps of the method of the first aspect.

The RFID device may be any one of: an access card, a credit card, adebit card, a pre-pay card, a loyalty card, an identity card, acryptographic card, or the like.

Certain preferred embodiments of the present invention will now bedescribed in greater detail, by way of example only and with referenceto the accompanying Figures, in which:

FIG. 1 illustrates a circuit for a prior art passive RFID device;

FIG. 2 illustrates a circuit for a passive RFID device incorporating afingerprint scanner; and

FIG. 3 illustrates an external housing for the passive RFID deviceincorporating the fingerprint scanner.

FIG. 2 shows the architecture of an RFID reader 104 and a passive RFIDdevice 102, which is a variation of the prior art passive RFID device 2shown in FIG. 1. The RFID device 102 shown in FIG. 2 has been adapted toinclude a fingerprint authentication engine 120.

The RFID reader 104 is a conventional RFID reader and is configured togenerate an RF excitation field using a reader antenna 106. The readerantenna 106 further receives incoming RF signals from the RFID device102, which are decoded by control circuits 118 within the RFID reader104.

The RFID device 102 comprises an antenna 108 for receiving an RF(radio-frequency) signal, a passive RFID chip 110 powered by theantenna, and a passive fingerprint authentication engine 120 powered bythe antenna 108.

As used herein, the term “passive RFID device” should be understood tomean an RFID device 102 in which the RFID chip 110 is powered only byenergy harvested from an RF excitation field, for example generated bythe RFID reader 118. That is to say, a passive RFID device 102 relies onthe RFID reader 118 to supply its power for broadcasting. A passive RFIDdevice 102 would not normally include a battery, although a battery maybe included to power auxiliary components of the circuit (but not tobroadcast); such devices are often referred to as “semi-passive RFIDdevices”.

Similarly, the term “passive fingerprint/biometric authenticationengine” should be understood to mean a fingerprint/biometricauthentication engine that is powered only by energy harvested from anRF excitation field, for example an RF excitation field generated by theRFID reader 118.

The antenna comprises a tuned circuit, in this arrangement including aninduction coil and a capacitor, tuned to receive an RF signal from theRFID reader 104. When exposed to the excitation field generated by theRFID reader 104, a voltage is induced across the antenna 108.

The antenna 108 has first and second end output lines 122, 124, one ateach end of the antenna 108. The output lines of the antenna 108 areconnected to the fingerprint authentication engine 120 to provide powerto the fingerprint authentication engine 120. In this arrangement, arectifier 126 is provided to rectify the AC voltage received by theantenna 108. The rectified DC voltage is smoothed using a smoothingcapacitor and supplied to the fingerprint authentication engine 120.

The fingerprint authentication engine 120 includes a processing unit 128and a fingerprint reader 130, which is preferably an area fingerprintreader 130 as shown in FIG. 3. The fingerprint authentication engine 120is passive, and hence is powered only by the voltage output from theantenna 108. The processing unit 128 comprises a microprocessor that ischosen to be of very low power and very high speed, so as to be able toperform biometric matching in a reasonable time.

The fingerprint authentication engine 120 is arranged to scan a fingeror thumb presented to the fingerprint reader 130 and to compare thescanned fingerprint of the finger or thumb to pre-stored fingerprintdata using the processing unit 128. A determination is then made as towhether the scanned fingerprint matches the pre-stored fingerprint data.In a preferred embodiment, the time required for capturing a fingerprintimage and accurately recognising an enrolled finger is less than onesecond.

If a match is determined, then the RFID chip 110 is authorised totransmit a signal to the RFID reader 104. In the FIG. 2 arrangement,this is achieved by closing a switch 132 to connect the RFID chip 110 tothe antenna 108. The RFID chip 110 is conventional and operates in thesame manner as the RFID chip 10 shown in FIG. 1 to broadcast a signalvia the antenna 108 using backscatter modulation by switch on and off atransistor 116.

FIG. 3 shows an exemplary housing 134 of the RFID device 102. Thecircuit shown in FIG. 2 is housed within the housing 134 such that ascanning area of the fingerprint reader 130 is exposed from the housing134.

Prior to use the user of the RFID device 102 must first enroll hisfingerprint date onto a “virgin” device, i.e. not including anypre-stored biometric data. This may be done by presenting his finger tothe fingerprint reader 130 one or more times, preferably at least threetimes and usually five to seven times. An exemplary method of enrolmentfor a fingerprint using a low-power swipe-type sensor is disclosed in WO2014/068090 A1, which those skilled in the art will be able to adapt tothe area fingerprint sensor 130 described herein.

The housing may include indicators for communication with the user ofthe RFID device, such as the LEDs 136, 138 shown in FIG. 3. Duringenrolment, the user may be guided by the indicators 136, 138, which tellthe user if the fingerprint has been enrolled correctly. The LEDs 136,138 on the RFID device 102 may communicate with the user by transmittinga sequence of flashes consistent with instructions that the user he hasreceived with the RFID device 102.

After several presentations, the fingerprint will have been enrolled andthe device 102 may be forever responsive only to its original user.

With fingerprint biometrics, one common problem has been that it isdifficult to obtain repeatable results when the initial enrolment takesplace in one place, such as a dedicated enrolment terminal, and thesubsequent enrolment for matching takes place in another, such as theterminal where the matching is required. The mechanical features of thehousing around each fingerprint sensor must be carefully designed toguide the finger in a consistent manner each time it is read. If afingerprint is scanned with a number of different terminals, each onebeing slightly different, then errors can occur in the reading of thefingerprint. Conversely, if the same fingerprint sensor is used everytime then the likelihood of such errors occurring is reduced.

As described above, the present device 102 includes a fingerprintauthentication engine 120 having an onboard fingerprint sensor 130 aswell as the capability of enrolling the user, and thus both the matchingand enrolment scans may be performed using the same fingerprint sensor130. As a result, scanning errors can be balanced out because, if a usertends to present their finger with a lateral bias during enrolment, thenthey are likely to do so also during matching.

Thus, the use of the same fingerprint sensor 130 for all scans used withthe RFID device 102 significantly reduces errors in the enrolment andmatching, and hence produces more reproducible results.

In the present arrangement, the power for the RFID chip 110 and thefingerprint authentication engine 120 is harvested from the excitationfield generated by the RFID reader 104. That is to say, the RFID device102 is a passive RFID device, and thus has no battery, but instead usespower harvested from the reader 104 in a similar way to a basic RFIDdevice 2.

The rectified output from second bridge rectifier 126 is used to powerthe fingerprint authentication engine 120. However, the power requiredfor this is relatively high compared to the power demand for thecomponents of a normal RFID device 2. For this reason, is has notpreviously been possible to incorporate a fingerprint reader 130 into apassive RFID device 102. Special design considerations are used in thepresent arrangement to power the fingerprint reader 130 using powerharvested from the excitation field of the RFID reader 104.

One problem that arises when seeking to power the fingerprintauthentication engine 120 is that typical RFID readers 104 pulse theirexcitation signal on and off so as to conserve energy, rather thansteadily emitting the excitation signal. Often this pulsing results in aduty cycle of useful energy of less than 10% of the power emitted bysteady emission. This is insufficient to power the fingerprintauthentication engine 120.

RFID readers 104 may conform to ISO/IEC 14443, the internationalstandard that defines proximity cards used for identification, and thetransmission protocols for communicating with them. When communicatingwith such RFID devices 104, the RFID device 102 can take advantage of acertain feature of these protocols, which will be described below, toswitch the excitation signal from the RFID reader 104 to continuous forlong enough to perform the necessary calculations.

The ISO/IEC 14443-4 standard defines the transmission protocol forproximity cards. ISO/IEC 14443-4 dictates an initial exchange ofinformation between a proximity integrated circuit card (PICC), i.e. theRFID device 102, and a proximity coupling device (PCD), i.e. the RFIDreader 104, that is used, in part, to negotiate a frame wait time (FWT).The FWT defines the maximum time for PICC to start its response afterthe end of a PCD transmission frame. The PICC can be set at the factoryto request an FWT ranging from 302 μs to 4.949 seconds.

ISO/IEC14443-4 dictates that, when the PCD sends a command to the PICC,such as a request for the PICC to provide an identification code, thePCD must maintain an RF field and wait for at least one FWT time periodfor a response from the PICC before it decides a response timeout hasoccurred. If the PICC needs more time than FWT to process the commandreceived from the PCD, then the PICC can send a request for a wait timeextension (S(WTX)) to the PCD, which results in the FWT timer beingreset back to its full negotiated value. The PCD is then required towait another full FWT time period before declaring a timeout condition.

If a further wait time extension (S(WTX)) is sent to the PCD beforeexpiry of the reset FWT, then the FWT timer is again reset back to itsfull negotiated value and the PCD is required to wait another full FWTtime period before declaring a timeout condition.

This method of sending requests for a wait time extension can be used tokeep the RF field on for an indefinite period of time. While this stateis maintained, communication progress between the PCD and the PICC ishalted and the RF field can be used to harvest power to drive otherprocesses that are not typically associated with smart cardcommunication, such as fingerprint enrolment or verification.

Thus, with some carefully designed messaging between the card and thereader enough power can be extracted from the reader to enableauthentication cycle. This method harvesting of power overcomes one ofthe major problem of powering a passive fingerprint authenticationengine 120 in a passive RFID device 102, particularly for when afingerprint is to be enrolled.

Furthermore, this power harvesting method allows a larger fingerprintscanner 130 to be used, and particularly an area fingerprint scanner130, which outputs data that is computationally less intensive toprocess.

As discussed above, prior to use of the RFID device 102, the user of thedevice 102 must first enroll themself on the “virgin” device 102. Afterenrolment, the RFID device 102 will then be responsive to only thisuser. Accordingly, it is important that only the intended user is ableto enroll their fingerprint on the RFID device 102.

A typical security measure for a person receiving a new credit or chipcard via the mail is to send the card through one mailing and a PINassociated with the card by another. However for abiometrically-authenticated RFID device 102, such as that describedabove, this process is more complicated. An exemplary method of ensuringonly the intended recipient of the RFID device 102 is able to enrolltheir fingerprint is described below.

As above, the RFID device 102 and a unique PIN associated with the RFIDdevice 102 are sent separately to the user. However, the user cannot usethe biometric authentication functionality of the RFID card 102 until hehas enrolled his fingerprint onto the RFID device 102.

The user is instructed to go to a point of sale terminal which isequipped to be able to read cards contactlessly and to present his RFIDdevice 102 to the terminal. At the same time, he enters his PIN into theterminal through its keypad.

The terminal will send the entered PIN to the RFID device 102. As theuser's fingerprint has not yet been enrolled to the RFID device 102, theRFID device 102 will compare the keypad entry to the PIN of the RFIDdevice 102. If the two are the same, then the card becomes enrollable.

The card user may then enroll his fingerprint using the method describedabove. Alternatively, if the user has a suitable power source availableat home, he may take the RFID device 102 home and go through a biometricenrolment procedure at a later time.

The RFID device 102, once enrolled may then be used contactlessly usinga fingerprint, with no PIN, or with only the PIN depending on the amountof the transaction taking place.

I claim:
 1. A method for harvesting power in a passive RFID devicecomprising a passive biometric authentication engine, the methodcomprising: receiving, by the RFID device, a command from a powered RFIDreader; receiving, by the RFID device, a non-pulsing continuousradio-frequency excitation field whilst the RFID reader waits for aresponse to the command; harvesting, by the RFID device, power from theexcitation field; supplying the power extracted from the excitationfield to the biometric authentication engine; performing a process inthe biometric authentication engine, the process being one not requiredfor responding to the command from the RFID reader; determining a periodthat the RFID device has been waiting for a response; and responsive todetermining that the period exceeds a predetermined threshold, if theprocess has not been completed, sending, by the RFID device, a requestfor a wait time extension to the RFID reader.
 2. A method according toclaim 1, wherein the RFID device does not respond to the command whilstthe biometric authentication engine is performing the process.
 3. Amethod according to claim 1, further comprising: after the biometricauthentication engine completes the process, responding by the RFIDdevice to the command.
 4. A method according to claim 1, wherein thesteps of determining a period and sending a request for a wait timeextension to the RFID reader are repeated until the process is completedand/or a response to the command has been sent.
 5. A method according toclaim 1, wherein the period is a time since the command was received orsince the last wait time extension request was made.
 6. A methodaccording to claim 1, wherein the process performed by the biometricauthentication engine is a biometric enrolment process.
 7. A methodaccording to claim 1, wherein the process performed by the biometricauthentication engine is a biometric matching process.
 8. A methodaccording to claim 1, wherein the process requires greater than 5.0seconds to be completed.
 9. A method according to claim 1, wherein thebiometric authentication engine includes a biometric scanner and aprocessing unit.
 10. A method according to claim 9, wherein thebiometric authentication engine is a fingerprint authentication engineand the biometric scanner is a fingerprint scanner.
 11. A methodaccording to claim 1, wherein the RFID device is a proximity integratedcircuit card (PICC) and the RFID reader is a proximity coupling device(PCD).
 12. A method according to claim 11, wherein the predeterminedthreshold is less than a first wait time (FWT) of the PCD.
 13. A passiveRFID device comprising: an antenna for receiving a radio-frequencyexcitation field from an RFID reader and for harvesting power from theexcitation field; a passive biometric authentication engine arranged toreceive power harvested by the antenna; and an RFID device controllerarranged to perform a method, comprising: receiving, by the antenna, acommand from a powered RFID reader; receiving, by the antenna, asubstantially continuous radio-frequency excitation field whilst theRFID reader waits for a response to the command; performing a process inthe biometric authentication engine, the process being one not requiredfor responding to the command from the RFID reader; determining a periodthat the RFID device has been waiting for a response; and responsive todetermining that the period exceeds a predetermined threshold if theprocess has not been completed, sending by the antenna a request for await time extension to the RFID reader.
 14. A passive RFID deviceaccording to claim 13, wherein the RFID controller is configured not torespond to the command whilst the biometric authentication engine isperforming a process.
 15. A passive RFID device according to claim 13,wherein the method further comprises: after the biometric authenticationengine completes the process, responding by the RFID device to thecommand.
 16. A passive RFID device according to claim 13, wherein thesteps of determining a period and sending a request for a wait timeextension to the RFID reader are repeated until the process is completedand/or a response to the command has been sent.
 17. A passive RFIDdevice according to claim 13, wherein the period is a time since thecommand was received or since the last wait time extension request wasmade.
 18. A passive RFID device according to claim 13, wherein theprocess performed by the biometric authentication engine is a biometricenrolment process.
 19. A passive RFID device according to claim 13,wherein the process performed by the biometric authentication engine isa biometric matching process.
 20. A passive RFID device according toclaim 13, wherein the process requires greater than 5.0 seconds to becompleted.
 21. A passive RFID device according to claim 13, wherein thebiometric authentication engine includes a biometric scanner and aprocessing unit.
 22. A passive RFID device according to claim 21,wherein the biometric authentication engine is a fingerprintauthentication engine and the biometric scanner is a fingerprintscanner.
 23. A passive RFID device according to claim 13, wherein thepassive RFID device is a proximity integrated circuit card (PICC) andthe RFID reader is a proximity coupling device (PCD).
 24. A passive RFIDdevice according to claim claim 13, wherein the predetermined thresholdis set to be less than a first wait time (FWT) of the PCD.